The present invention relates generally to medical devices and, more particularly, to a system and method for predictively controlling thermal output of a magnetic resonance (MR) imaging device.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. That is, active electric coils are used to drive the spatial gradients into the static magnetic B0 field. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Some imaging processes demand higher amplitude gradient fields, faster field transitions, and greater duty cycles to improve image resolution, contrast, and scan time. These processes, often referred to as enhanced imaging processes, typically require more power than non-enhanced processes and result in higher thermal dissipation within the MR imaging device. As such, for a given enhanced imaging process, the resulting thermal output of the MR imaging device may exceed desirable limits and, thus, it may not be possible to acquire data using the given process. Or, in some systems, a given scan may be interrupted as a result of an undesirable thermal dissipation.
That is, some MR imaging devices have a bore temperature monitoring system (BTMS) to monitor the temperature inside the bore. Specifically, the BTMS halts operation of the MR imaging device if a temperature in the bore has surpassed a desired level or limit. As such, the BTMS dynamically interrupts the scan to allow the temperature in and around the MR imaging device and, specifically, the bore, to decrease to the desirable level.
Relying solely on a BTMS to regulate the imaging process has some drawbacks. For example, a BTMS is a reactionary tool that only halts a given scan after bore temperature exceeds a given threshold. Furthermore, periodically halting operation of the MR imaging device to allow the bore temperature to drop injects undesirable delays into scheduled imaging processes and, as a result, increases scan time and negatively affects throughput. Accordingly, systems have been developed to avoid repeated engagement of the BTMS by setting a constant limit on a basic operating parameter of the MR imaging device, for example, coil current. That is, some MR imaging devices include software to limit temperatures by holding the root-mean-square (RMS) current levels used to generate the gradient fields to a predefined value.
Accordingly, these systems preclude prescription of enhanced imaging processes that would cause temperature levels to rise above desirable limits. However, by fixing the peak power supplied to a coil, these systems ignore the temporal response of the specific MR imaging device. That is, these systems, which employ hard limits, rely on assumptions concerning the use profile and boundary conditions of the MR imaging device and do not consider the actual thermal output of the MR device for a particular imaging process. Further, the assumptions concerning the use profile and boundary conditions of the MR imaging device are generally conservative so as to ensure that the MR imaging device is not driven to produce excessive temperatures.
As such, these predefined hard thresholds often limit the MR imaging device from performing many processes that the MR imaging device is otherwise capable of executing without producing excessive temperatures. That is, the MR imaging device is often conservatively controlled to not execute a given scan notwithstanding that the resulting thermal output would still remain below thermal limits. Specifically, because these predefined hard limits are based on quantities such as gradient current or power, they do not consider the actual temperatures in and around the MR imaging device, the frequency of the desired imaging process, the axes selection included in the desired imaging process, or the specific coils employed during the desired imaging process. Accordingly, the predefined limits, such as current limits, often restrict an operator from utilizing enhanced, or more aggressive, scanning procedures even through the associated temperatures would remain within acceptable limits. As a result, the diagnostic capability of the MR imaging is unnecessarily restricted.
It would therefore be desirable to have a system and method capable of dynamically controlling thermal output of a medical device. In particular, it would be desirable to have a system and method capable of predicting a thermal output of a medical device based on particulars of an impending use of the device and dynamically controlling the medical device substantially consistent with the particulars while maintaining the thermal output of the medical device within a desired temperature range. It would also be desirable to have a method and system to control power to coils of an MR device during execution of an enhanced imaging process without driving a bore temperature to a level in excess of a given value.